Revise guidance around end and done pages

* Revise guidance to previous vesions around end pages
  * This reverts commit 779abdf2034c9c412f4d76d4bfc601dfcd1f020c.
  * This reverts commit 263396b8506e4ce4afea6fb2bd16b5811e810bca.
* Add detail around done pages distinct from end pages

Contact Subhi Barakat for details on why and how these changes were made.
It's best to talk about the GOV.UK Support form, rather than Zendesk here. It might also be good to link to [the guidance on how to contact GDS](https://www.gov.uk/guidance/contact-the-government-digital-service/how-to-contact-gds).